German chemical giant Bayer has agreed to a $66bn (£50bn) deal to buy controversial US agrochemical giant Monsanto and create the world's largest seeds and pesticides company. The proposed deal, the biggest corporate takeover deal so far this year, follows a wave of consolidation in the seeds and agriculture industry and has raised concerns among scientists, regulators, farmers and activists who called the deal a "marriage made in hell".

Werner Baumann, chief executive of Bayer, which is most famous for developing aspirin, said "the combination of our two great organizations [will] deliver substantial value to shareholders, our customers, employees and society at large". But farmers and environmentalists warned the deal could lead to a reduction in seed variety, an increase in genetically modified crops and higher seed costs and therefore crop and food prices.

The proposed takeover is likely to face intense regulatory scrutiny in the US and Europe, particularly as it quickly follows two other mega-deals in the agriculture industry and would leave control of almost two-thirds of the world's seeds and pesticides in the hands of three firms.

Analysts at Bernstein Research said they thought there was only a 50:50 chance of the deal winning regulatory clearance. "We believe political pushback to this deal, ranging from farmer dissatisfaction with all their suppliers consolidating in the face of low farm net incomes to dissatisfaction with Monsanto leaving the United States, could provide significant delays and complications", they said in a research note. Because of the difficulties expected in getting the deal through, Bayer has agreed to pay Monsanto $2bn if the tie-up falls apart because of competition concerns.

Friends of the Earth described the takeover, which will see Bayer pay $128 per share – a 44% premium on Monsanto's share price before the proposed deal was first revealed in May, as a "marriage made in hell".